1. Field of the Invention
The present invention relates to an electromagnetic energy seal for a microwave oven, and particularly to an electromagnetic energy seal for a microwave oven, which can effectively prevent the leakage of electromagnetic energy through a gap between a front plate and a door of the microwave oven.
2. Description of the Prior Art
Conventionally, a combination of a choke seal and a capacitative seal has been used, to prevent the leakage of electromagnetic energy through the gap between the front plate and the door of the microwave oven.
Referring to FIGS. 1 and 2, a conventional electromagnetic energy seal is shown, wherein a choke channel 4 is defined in the interior having a door frame 3 of a ".hoarfrost."-type cross section, which frame is disposed adjacent to the outer peripheral edge of a door 2 of a microwave oven. On the inner wall 3a of the door frame 3, a seal plate 6 is mounted parallel to a front plate 5 of the oven body 1 to form an opening 7 with a certain width between the door frame 3 and the seal plate 6. The width is determined such that the distance between the inner wall 3a of door frame 3 and the center line 7a of the opening 7 is .lambda./4 (herein, .lambda. is the wavelength of the electromagnetic energy adapted to heat the food). A gap 8 is formed between the front plate 5 and the seal plate 6.
In the conventional electromagnetic energy seal of the above-mentioned construction, the inner wall 3a of the choke channel 4 functions as a short circuit plane with respect to the electromagnetic energy being leaked outwardly through the gap 8 from the heating room of the oven, so that the impedance of the inlet 8a of gap 8 is very low, thereby causing the electromagnetic energy to be reflected from the inlet 8a.
For example, in the case of a waveguide, the impedance Z.sub.L at a certain distance d is represented as follows: EQU Z.sub.L =jZ.sub.O tan (2.pi.d)/.lambda.
wherein, j is .sqroot.-1 and Z.sub.O is the characteristic impedance.
In the above equation, when the distance d is .lambda./4, the impedance Z.sub.L goes to the infinite value. On the other hand, when the distance d is .lambda./2, the impedance Z.sub.L becomes 0.
Accordingly, since the distance between the inner wall 3a of the door frame 3 and the center line 7a of the opening 7 is about .lambda./4 and the distance between the center line 7a and the inlet 8a is about .lambda./4 in the above-mentioned construction, so that the distance between the inner wall 3a and the inlet 8a is about .lambda./2, the impedance at the inlet 8a becomes close to 0. As a result, the electromagnetic energy is reflected from the inlet 8a, so that the leakage of the electromagnetic energy through the gap 8 can be avoided. And also, the gap 8 between the front plate 5 and the seal plate 6 functions as a capacitive seal having a low impedance with respect to the electromagnetic energy, thereby preventing the leakage of the electromagnetic energy.
In such conventional electromagnetic energy seal, however, the following problems occur:
(1) When the front plate 5 and the seal plate 6 make contact with each other at the point P to form a metal-to-metal contact point, the metal-to-metal contact point P functions as a short circuit point. As a result, the impedance of the inlet 8a of the gap 8 can not be reduced, and thus, the choke seal can not be formed, so that the leakage of the electromagnetic energy can not be completely prevented.
(2) The above-mentioned effect of the choke seal is sharply reduced, as the width of the gap 8 increases. This may be apparent from a laboratory test. Generally, the characteristic impedance of the parallel transmission line formed by the front plate 5 and the seal plate 6 is inversly proportional to the width of the gap 8. For example, as the width of the gap 8 is increased from 50 .mu.m to 1 mm, by 20 times, the characteristic impedance is reduced by 1/20 times, as compared with the case of 50 .mu.m.
On the other hand, in order to prevent the generation of a spark between the seal plate 6 and the front plate 5, an insulation film with a thickness of about 50 .mu.m is attached, or an oxide film is formed on the seal plate 6 or the front plate 5. Various dimensions including the depth of the choke channel 4 are determined by the parallel transmission line with a length of .lambda./4 formed by the door frame 3 and the seal plate 6. Generally, the density of the leaked electromagnetic energy is determined to be at a minimum, when the width of the gap 8 is about 50 .mu.m and the parallel transmission line having a length of .lambda./4 is connected.
Therefore, even when these two .lambda./4 paths have different characteristic impedances, the dimensions of the choke system is determined to exhibit a maximum effect under the above-mentioned conditions. Thus, the change of the width of the gap 8 between the front plate 5 and the seal plate 6 causes the characteristic impedance of the transmission line to be changed. In order to prevent the reduction of the choke seal effect, consequently, the width of the gap 8 should be accurately and firmly maintained, when the door is mounted to the microwave oven. However, the width of the gap 8 is gradually increased, due to the looseness of the door hinge caused by the prolonged use thereof, so that the leakage of the electromagnetic energy is increased.
(3) The choke seal of the above-mentioned construction functions effectively when the electromagnetic energy enters at a right angle with respect to the choke channel 4. On the other hand, when the electromagnetic energy enters at an angle other than a right angle, for example 45.degree., with respect to the choke channel 4, the width-wise wavelength of the choke channel 4 becomes .sqroot.2.lambda., so that the effect of the choke seal is greatly reduced. The electromagnetic energy coming into the choke channel 4 has a rectangular component and a parallel component with respect to the longitudinal direction of the choke channel 4. The choke seal cannot be effective against the parallel component of the electromagnetic energy. Consequently, the above-mentioned construction has a disadvantage in that the leakage of the electromagnetic energy coming into the choke channel 4 at an incline can not be prevented.